1. Field of the Invention
The present invention relates to laminated glass face ply assemblies incorporating integral heating elements for aircraft windshields. More particularly, the present invention provides an outer protective laminate of thin chemically strengthened glass, or highly cross-linked acrylic plies laminated to the acrylic surface of the aircraft windshield structural layer by a thin urethane interlayer. The outer protective laminate incorporates multiple thin glass layers with thin urethane or other adhesive interlayers configured to incorporate braids or leads for an electrical anti-ice element without structural degradation.
2. Prior Art
Aircraft windows and windshields are typically fabricated using acrylic plastics in homogenous or laminated form or acrylic and glass laminated structures to meet weight and impact resistance requirements. In many of these window configurations, the external layer or surface comprises stretched acrylic plastic. Stretched acrylic provides a tough, reasonably durable finish, however, crazing, pitting, erosion and discoloration of the acrylic surface can be caused by particulates, hail and chemical attack. The toughness gained in stretching the acrylic reduces surface hardness thereby increasing the susceptibility to degradation.
Windshield laminates typically comprise one or more structural layers of glass, stretched acrylic, polycarbonate or other light weight plastic over which a glass face ply of approximately 0.125 inches minimum is laminated. A polyvinyl butyryl (PVB) or other adhesive interlayer of approximately 0.10 to 0.15 inches is employed. In more complex windshields the outer glass face ply is heated using an electrical element for anti-icing. The anti-ice element is applied to the inner surface of the face ply and conductive braid or other conductor is provided for supplying electrical power to the anti-ice element. The braid is typically accommodated within the interlayer thickness thereby avoiding any structural impact to the laminated windshield.
In recent years a worsening problem with aircraft windows has been created by significant volcanic eruptions which not only introduce significant quantities of high particulate ash into the upper atmosphere but also create a corrosive chemical environment severely degrading acrylic aircraft windows. Severe degradation of cabin windows for commercial airline aircraft is exemplary of this problem. Most regular airline travelers can attest to the significant degradation of visual quality in aircraft windows. The windows are crazed and/or pitted, significantly impairing vision. Studies have indicated that chemical attack from volcanically introduced substances at higher altitudes is the major contributing cause to such erosion.
In prior industry practice, airline cabin windows were removed and the exterior surfaces were ground and polished to restore the visual quality of the windows. Typical aircraft window configurations such as those shown in FIGS. 1A and 1B of the drawings had initial representative thicknesses of approximately 0.350.+-.0.015 inches with a minimum structural thickness requirement of 0.280 inches. This provided approximately 0.070 inches for grinding and polishing of the windows during refurbishment. This tolerancing allowed approximately three refurbishments of the window before reaching the minimum structural thickness. This machine grinding and polishing operation depends on flattening these relatively thin panes during the process using vacuum tooling or other means. The much thicker laminated cockpit side windows do not lend themselves to being flattened, so they are limited to less effective hand polishing and premature replacement if the surface damage is too deep to be removed in this manner.
Current atmospheric conditions are resulting in rapid degradation of windows and more frequent repair of the windows as described, which, along with more frequent replacement of aircraft windows due to increased frequency of repair, has created an extensive cost burden for commercial carriers. Consequently, modifications to original windows and a means to accomplish repair of acrylic aircraft windows to reduce damage and eliminate repetitive repair is sought.
Windshield laminates which require significant hail resistance typically employ a relatively thick glass face ply to meet excessive deflection and breakage requirements. In addition, the interlayer thickness of prior art windshields has been determined, at least in part, to accommodate the conductive braid or other contacts for anti-icing elements. A thinner outer face ply would be structurally feasible if a thinner interlayer could be employed to prevent excess deflection during an impact. However, the requirement for anti-ice provisions has not previously been accommodated with a thin face ply laminated system due to the requirement for thin interlayer thickness which will not accommodate the braid or other electrical conductor providing power to an anti-ice element. Machine relief in the underlying polycarbonate/acrylic structural layer to accommodate the braid would produce unacceptable stresses that would compromise bird impact resistance unless significant thickness increases were made in the structural layer thereby obviating the benefit of the thin face ply.